Stable seismic wave amplifier with automatic volume control



Dec. 1, 1942. H. HOOVER, JR

STABLE SEISMIC WAVE AMPLIFIER WITH AUTOMATIC VOLUME CONTROL Filed July 28, 1939 IN VEN TOR. limes/er [foo v52 JR.

A TTORNEYS.

Patented Dec. 1, 1942 STABLE SEISMIC WAVE ER WIT AUTOMATIC VOLUME CONTROL Herbert Hoover, In, Sierra Madre, CaliL, assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application July 28, 1939, Serial No. 287,072

' 6 Claims.

The present invention pertains to stable amplifiers using automatic volume control (AVC) and in particular provides a means and method for maintaining stability in amplifiers using AVC over a wide range of amplifier gain.

My invention is particularly applicable to seismic prospecting as instability is very likely to occur in amplifiers used for this work, due to the fact that the pass bands of both the main amplifier and the associated automatic volume control circuit may overlap to some extent.

According to the prior art, amplifiers with automatic volume control use variable gain in the amplifier and fixed gain in the associated volume control circuit. At very high amplifier gain, instability may arise in such circuits especially where the main amplifier and the AVG circuits pass waves in a common frequency band. To prevent such instability at high gain, the gain in the associated automatic volume control circuit is customarily kept at a suitable low value so as to always maintain the all around gain below a critical value above which instability occurs. This method of maintaining stability at high levels of amplifier gain has the disadvantage that the sensitivity of the automatic volume control action is low at low levels of amplifier gain corresponding to large incoming signals.

My invention overcomes these difficulties by providing high automatic volume control circuit gain when the gain of the controlled amplifier is low and low automatic volume control circuit gain when the gain of the controlled amplifier is high. By varying the gain of an amplifier as an inverse function of amplifier output signal strength and the gain of associated automatic volume control circuit as a direct function of output signal strength, I maintain the all around gain through said amplifier and said automatic volume control circuit below a value at which instability may occur.

Some of the objects of the present invention are: to provide a stable automatic volume coninstability occurs, and to provide a seismic wave amplifier with. stable automatic volume control.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is -therefore to be understood that my method is a manner that the all around gain through the applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus, of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Fig. 1 is a wiring diagram. of an amplifier utilizing automatic volume control in accordance with my invention.

Fig. 2 is a wiring diagram of an alternate form of automatic volume control circuit utilizing the principles of my invention.

My'invention pertains to the stable operation of amplifiers in which the gain of the main amplifier channel and the gain of the automatic volume control circuit are both varied simultaneously, in such a manner that the amplifier and automatic volume control circuits cooperate to procure stable operation through a wide range of signal amplitude such as that normally encountered in seismic prospecting. I achieve this stability at both high and low levels of input signal strength by varying the gain of the amplifier and the gain of the automatic volume control circuit in an inverse manner.

As illustrated in the drawing, my apparatus utilizes four principal parts, a seismic wave receptor or signal source I, a. reproducer 2, a main amplifier 3, and an automatic volume control circuit 4. It is clear that if the all around gain through amplifier 3 and automatic volume control circuit a exceeds a critical value, instability will occur. Such instability may manifest itself in any one of many forms, such as oscillation, selective regeneration, motorboating, etc.

By varying the gains of the two circuits 3 and 4 in such a manner that the all around gain through the system is less than the aforemennal grid it of the tube II. A portion of the output signal appearing in the anode l8 circuit is applied to reproducer 2. Another portion of the output signal is applied to automatic volume control circuit 3, where it is impressed upon grid 26 of the amplifier tube 2! through the coupling condenser 25. Corresponding amplified signals appearing in the anode 28 circuit are applied to the grid 36 of triode 3? which is biased to cut ofi by battery 3t. In the circuit of plate 38 are two filters 40 and 45 which provide variable gain contrcl voltages at their respective output terminals 42 and 43. Additional filtering between tube 3'3 and grid 6 of tube 1 may be provided by resistor 70 and condenser H.

In the embodiment of my invention illustrated, the automatic volume control circuit 3 controls both its own'gain and the gain of the main amplifier channel to achieve stable automatic vol-' put of filter fill is impressed on gain control elements 6 and Q6 of amplifier tubes 7 and II. This control voltage operates in such a manner as to compress signals passing through amplifier 3.

The gain control voltage appearing in the output of filter M is fed back to an amplifying device 27] within the automatic volume control circuit 11 in such a manner as to expand the amplitude range of signals passing through control unit 5.

In the system shown in Fig. l, the gain of the amplifier 3 decreases and the gain of the automatic volume control circuit 4% increases with the strength of signals applied from source l. appropriate selections of tubes, circuit constants, and voltages, the all around amplifications may be'kept less than the critical value above which instability occurs.

In Fig. 2 I have shown an alternate form of automatic volume control circuit 5. In both Figs. 1 and 2 signals from the output of amplifier 3 are applied to said automatic volume control circuit through conductor 2a and the automatic volume control voltage is applied to a gain' control element of amplifier 3 through conductor 5d.

'In Fig. 2 a portion of the signals arriving over lead are applied to grid 26 of amplifier tube. 2'1.

Corresponding amplified signals appearing in the plate 28 circuit are applied to grid 36 of tube 31 biased to cutofi by battery 36. Rectified automatic volume control voltages appearing at point 42 are applied to gain control elements of put signals. The automatic volume control unit 4 of Fig. 2 expands as well as rectifies signals impressed thereon through lead 20. As in the case of the AVG circuit of Fig. 1 the all around gain through amplifier 3 and AVC circuit 4 may be kept below a critical value at which instability may occur.

Thus it is seen that I have provided for varying the gain of an automatic volume control circuit as an inverse function of the gain of the controlled amplifier. In this manner I am able to provide stable and sensitive automatic volume control action at both high and low levels of input signal strength.

Amplifiers operated in accordance with my invention have relatively fiat and stable response characteristics over a wide range of input signal strength and because of these characteristics my invention is especially valuable for use in seismic prospecting.

I claim:

1. In apparatus for seismic prospecting, an amplifier having an input and an output, and adapted to pass waves in a desired frequency range of seismic waves independently of the amplitude of waves being passed through the amplifier, an automatic volume control circuit interconnecting said output and a gain control element of said amplifier and adapted to respond to rapid changes in average output signal strength comparable in frequency to waves which are readily amplified by said amplifier,

and means for varying the gain of said automatic volume control circuit inversely as a function of the gain of said amplifier.

2. In apparatus for seismic prospecting, an

' amplifier having an input and an output, filtering means intermediatesaid input and said output adapted to pass waves in a desired frequency range of seismic waves independently of the amplitude of waves being passed through the amplifier, an automatic volume control circuit interconnecting said output and a gain control element of said amplifier and adapted to respond to rapid changes in average output signal strength comparable in frequency to waves which are readily amplified by said amplifier, means for varying the gain of said amplifier inversely as a function of output signal strength and means for simultaneously varying the gain of said amplifier directly as a function of output signal strength whereby the all around gain through said amplifier and said automatic volume control circuit is maintained below a value above which instability may occur.

3. In an amplifier having an input, an output, and a gain control element, filtering means intermediate said control element and said output providing said amplifier with a frequency vs. amplitude transmission characteristic which is independent of the amplitude of the signals transmitted through said amplifier, an automatic volume control circuit interconnecting said output and said control element, and having a frequency vs. amplitude characteristic which overlaps said filtering 'means characteristic, and

increase of the amplitude of the amplifier 3 outmeans for varying the gain of said amplifier and said automatic volume control circuit in .a mutually inverse relationship as a function of the amplitude of output signals while maintaining constant the frequency response 'of said filtering means.

4. In an amplifier having an input, an output, and a gain control element, filtering means intermediate said control element and said outasoasss put providing said amplifier with a frequency vs. amplitude transmission characteristic which is independent of the amplitude of the signals transmitted through said amplifier, an automatic volume control circuit interconnecting said output and said control element and having a frequency vs. amplitude characteristic which overlaps part ofsaid filtering means characteristic, and means ,for varying the gain of said automatic volume control circuit as an increasing Y .iunction' of amplifier output signal strength while maintaining constant the transmission characterisitic of said filtering means.

5. In an amplifier having an input, an output,

" and'again control element, filtering means in-v plifier, and having a frequency vs. amplitude characteristic which overlaps part of said filtering means characteristic, means for changing the gain of said amplifier in one direction in accordance with the output of the automatic volume control circuit while maintaining said filtering means transmission characteristic constant, and means for simultaneously changing the gain of said automatic volume control circuit in the opposite direction.

6. In an amplifier having an input, an output, and a gain control element, filtering means intermediate the gain control element and the output, an automatic volume control circuit connected between the output and the gain control element, the constants of the automatic volume control circuit being such that the gain of the amplifier may be varied at a frequency comparable to the frequencyoi waves readily transmitted through said filter, and means for varying the gain of said automatic volume control circuit inversely as a function of the amplifier gain.

WEB? HOOVER, JR. 

